Phase-shift circuit for grid-controlled gas discharge tubes



April 3, 1952 J. H. LUCAS 2,591,952

PHASE-SHIFT CIRCUIT FOR GRID-CONTROLLED GAS DISCHARGE TUBES Filed March 30, 1950 l 7 7 I2 I 6 e a 2 4 9 Q5 INVENTOR. JOHN HAROLD UCAS AGENT Patented Apr. 8, 1952 PHASE-SHIFT CIRCUIT FOR GRID-CON- TROLLED GAS DISCHARGE TUBES John Harold Lucas, Salfords, near Redhill, England, assignor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application March 30, 1950, Serial No. 152,829 In Great Britain May 20, 1949 The invention relates to phase shifting circuits employing direct current saturable reactors for the grid control of two or more electric discharge tubes.

It is known to use such circuits so that at desired instants at which conduction is to ensue rapid changes occur in the voltage which is applied to the grid or grids of the discharge tubes. This is a desirable characteristic for the control of electric discharge tubes having a gas of vapour filling, more especially when they are used in rectifier circuits or to supply an inductive load. In such circuits stable working may depend on the steepness of the grid control voltage wavefront during the said rapid changes.

The circuit diagram shown in Figure 1 of the accompanying drawings, shows simply, a known phase shifting circuit of the type referred to. A transformer I has its primary winding 2 supplied from a source of alternating voltage 3, while its secondary winding 4 is centre-tapped at 5. The voltage across winding 4 is applied across the series connection of one winding 6 of a transformer 1 and a resistance 8 having their junction at 9. A transformer 1 has a core cap-able of magnetic saturation by a winding Ii] which is supplied from a source of direct current ll, preferably through an inductance l2 (or a high resistance) so as to keep out alternating current from the circuit comprised by winding 10, inductance l2 and the source H.

The alternating voltage By that appears between points and 9 will have a certain phase relationship to that of the source 3, and it is well known that this phase may be shifted in angular degrees over a range of nearly 180 by varying the saturation of the core of transformer I by means of the source I I. An accompanying effect is that the wave-form of the voltage Eg is distorted with reference to the waveform of the source 3 due to non-linearity of the magnetic core 1, and if this core is of a material having a high permeability and a sharply defined saturation flux density, such as the material having the registered trade-mark Numetal, the waveform of the voltage Eg may be arranged to exhibit rapid changes which are nearly discontinuous.

When only one discharge tube has to be controlled it is achieved simply by connecting the cathode to point 5 and the grid to point 9 and arranging that the rapid change of voltage is such as to drive the grid positive at the desired instant of conduction. This instant may be varied 2 Claims. (Cl. 315-194) 2 i simply by varying the direct current supplied to winding It].

When two discharge tubes have to be controlled which are operated in bi-phase, it has hitherto been usual to obtain a voltage Eg across points 5 and 9 which is applied to the primary of a transformer having a centre tapped secondary winding the centre tap being joined to the oathodes of the discharge tubes, one end of the secondary winding joined to the control grid of one tube, the other end being joined to the control grid of the other tube. Apart from the transformer I, it hastherefore been necessary to use one transformer to obtain variable phase shift and another transformer to allow bi-phase operation and the design of these two transformers is such that they have generally been different in type.

The present invention seeks to obtain an equivalent result by using a circuit employing a plurality of identical transformers of the type of transformer l in Figure 1, having the direct current windings so connected as substantially to balance out'the alternating currents generated in them. The impedance in series with the D. C. source may then be of much reduced value or may be eliminated.

According to the present invention, a phaseshifting circuit for the grid control of discharge tubes operating in multi-phase relationship on a multi-phase alternating current supply,v has a saturable reactor connected in series with a resistance across each phase of the supply, the junction between the saturable reactor and the resistance of each pair being connected to supply a grid control potential to the corresponding discharge tube and each of the saturable reactors has a direct current winding for saturating the core of the reactor, connected in series across a variable direct current supply.

A phase-shifting circuit, according to the invention, for the grid control of two discharge tubes connected for bi-phase operation on an alternating current supply, has a bridge-connected network consisting of two saturable reactors in two opposite arms of the bridge and two resistances in the two adjacent opposite arms, two opposite points of the bridge network being connected across the alternating current supply and the other two opposite points of the bridge network being connected each to supply a grid control potential to the corresponding discharge tube and both the saturable reactors having a direct current winding for saturating the core of the reactor connected in series across the variable direct current supply.

An impedance, preferably, a choke, may be connected in series with the direct current windings of the saturable reactors and the variable direct current supply, in either of the arrangements described-above.

One ,circuit according to the ,present invention, employing two discharge tubes connected for bi-phase operation on an alternating current supply, will now be described by way of example, with reference to Figure -2'ofithe accompanying drawings.

In Figure 2, correspondingreference numerals to those used in Figure l 'are-iusedto denote-the corresponding function of ,parts described in Figure 2.

Two grid-controlled discharge tubesl3"*and t3 are connected for bi-phase operationfromthe alternating current supply 3, having their anodes connected to opposite ends of a centre-tapped secondarywinding 14 of a transformer l, whose primary winding '2 is connected across theialtei natingccurrent supply 3.

.A bridge-connected -.network .consists of two s-aturablereactors"! and 'i' having their reactive windings 6 and 6' connectedin two. opposite arms of the bridgenetwoi'k. Two resistances 8 vandt' are-connected in the two adj acentoppos'ite arms. Two opposite points i9 and I 9 ,of the bridg (network aresconnected to supplygrid control :poten- .tials to the two discharge tubes 3 and 13'. The ,point'9 is connected throughiagridresistance I5 to-the .gridofthe corresponding tube 13, that is, the-tube whoseanode potential is in therequired ,phase relationship to the potential appearing at ,pointvfi. ,The/pointBisconnected to the grid of .the..corresponding discharge tube t3 through a grid resistance !5. The other two opposite ipoints inthe bridge network are connected to the router ends of the centre-tapped winding 4 of transformer Lthecentre point-5 being-connected tothe cathodes of the two tubes 13 and I3.

The-direct currentwindings IE1 and-lfl of the :saturable reactors! and J'are connected in series with :each other and in series with a choke 12 across a variable direct current supply H.

As with the circuit of Figure l, thecontrol of the discharge tubes'in Figure 2 is-efiected by varying the :magnitude of the current flowing through the direct current windings 4-5] and iii of the saturable reactors 1 and'l". change of th potential .at the points -9 and 9' varies correspondingly in relation to the anode motentlalof the corresponding valve so that the variation-ref the direct=currentsupply H controls the twotubes Hand [3 in a similarmanner.

Since the direct current windings it] and It The phase 4 are connected in series, the alternating potential induced in each of them cancels out approximately so that only a very small alternating potential appears across the variable direct current supply l I. Anyv residual potential. is suppressed .by the'inclusion'of the'choke [2 described in connection with this example. In this case, the

vchoke l2 need have only a much smaller impedance than the choke l2 in Figure 1 and may be eliminated altogether if the direct current supply i I has a sufficiently high internal impedance.

The advantage of the circuit shown in Figure 2 is that the...transformers l are simple two winding types, identicalin construction while, due to the direct connection of the grids to the phase shifting elements, there is not the load on these elementsthat would occur if a phase transformer were interposed. The waveform of the grid voltage has the same desirable properties as that obtained from the circuit shown in Figure .1.

'WhatI claim is:

1. Phase-shifting apparatus for the control of two grid-controlled discharge tubesconnectediin a two phase circuit to an alternatingicurrent supply, said apparatus comprising a .pair ,ofsatur able reactors 'having a direct-current winding, the direct-current windings of said reactors'being connected. in series, a pair of resistances, a fourvarmibr'idge network, onepair of opposing arms of said .networkbeing constituted by-said reactors, the other pair of opposingiarmsIbeing constituted .bysaid resistances, means to apply a vvariable direct-current to said serially connected windings, .said windings being .4 connected serially in aadirection at which the-alternating currents induced therein by said reactors are substantially vbalanced out, means connecting onepair of vertices of said bridge network-to said alternatingecurrent supply, and means connected to theother pair of verticesto the control grids of saidtubes.

.2. Apparatus, as set forth in'claim -.l, further including an impedance in the pathof said variable direct-current.

- JOHN 'HAROLD LUCAS.

REFERENCES CITED The following references are of record in :the file of this patent:

UNITED STATES PATENTS Number Name Date 1,926,275 Fitzgerald Sept. 12, 1933 1,943,372 Fitzgerald Feb. 20, 1934 2,246,181 Morton June 17,1941 

